As others have mentioned - check that front sprocket nut. With just 20k on it, it may well still have original chain/sprockets. Check if it's been swapped for thicker nut, check it hasn't fallen off in past and now bodged/welded up onto drive shaft. Anything suspect here and just walk away from the bike - it won't be worth it. Even if all good, if it's original chain you'll likely need to replace that before it hits 30k (£160 ish parts & labour)
Shock will likely be original and in need of replacement soon (budget £100-150 to refurb, or £300 for new Hagon shock).
Check rear caliper - often gets clogged up. Many swap it for FZS1000 blue spot caliper which is more reliable (budget £60-100 parts)
Check downpipes as bottom underneath. All original ones rust through - even my much later 2003 models wa full of holes by 20k odd (budget £200 parts + labour. Couldbe tricky job if studs on engine snap).
Check if brake lines are still orginal rubber ones - if so, budget on replacing with braided lines at this age (£80 parts). Brake fluid is probably long overdue a change anyway.
As with any old bike, check everything else, all lights and electrics work, kill switch/side stand cuts out engine. Check for leaking oil on fork seals (£100-150 job). Check oil has no cream goo in it (sight glass on bottom right of engine), or smoke from oil filler cap when rev'd gently - that'll be screwed engine, so walk away. Check all big fat water hoses aren't cracked/split with age - there's no cheap aftermarket silicone hoses available, genuine hoses are about £20-30 each.
As it's been sat for whilte, check it rev's well, carbs may need a clean (lots of labour). Check age on tyres (google how to read tyre manufacture date), they could be brittle death traps by now, just budget on £200-250 to replace immediately. Check there's no rust inside the tank. See when coolent was last changed - it could need a thorough flush through now (1h labour).
TBH if you're mate actually going to properly service the bike, or just shove it through an MOT alone? If only the latter, I'd plan on taking it straight to a decent garage who will do a thorough job changing all fluids/filters etc (budget £200-300).